Varifocal structure comprising a liquid lens structure in optical series with a liquid crystal lens in a head-mounted display and method of adjusting an optical power of the varifocal structure

ABSTRACT

A varifocal block includes liquid crystal (LC) lens and a liquid lens structure in optical series. The LC lens has a plurality of optical states, including an additive state that adds optical power to the LC lens and a subtractive state that removes optical power from the LC lens. The liquid lens structure comprises a transparent substrate layer, a deformable membrane, and a volume of liquid enclosed between the transparent substrate layer and the deformable membrane. The deformable membrane has an adjustable range of optical power dependent on an adjustable curvature of the deformable membrane. The plurality of optical states of the LC lens and the adjustable range of optical power of the liquid lens structure together provide a continuous range of optical power for the varifocal block.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/693,839, filed Sep. 1, 2017, which claims the benefit of U.S.Provisional Application No. 62/423,091, filed Nov. 16, 2016, which isincorporated by reference in its entirety.

BACKGROUND

The present disclosure generally relates to enhancing images fromelectronic displays, and specifically to varying a focal length ofoptics with a compact design to enhance comfortable viewing experiencein head mounted displays.

Virtual reality (VR) headset can be used to simulate virtualenvironments. For example, stereoscopic images can be displayed on anelectronic display inside the headset to simulate the illusion of depthand head tracking sensors can be used to estimate what portion of thevirtual environment is being viewed by the user. Such a simulation,however, can cause visual fatigue and nausea resulting from an inabilityof existing headsets to correctly render or otherwise compensate forvergence and accommodation conflicts. Augmented Reality (AR) headsetsdisplay a virtual image overlapping with the real world. To createcomfortable viewing experience, the virtual image generated by the ARheadsets needs to be displayed at the right distance for the eyeaccommodations of the real world objects at different time.

SUMMARY

A varifocal block has a continuous range of adjustment of optical power.The varifocal block includes a liquid crystal (LC) lens and a liquidlens structure. The LC lens has a plurality of optical states thatinclude an additive state that adds optical power to the LC lens and asubtractive state that removes optical power from the LC lens. Theliquid lens structure is in optical series with the LC lens. The liquidlens structure includes a transparent substrate layer and a deformablemembrane. The deformable membrane has an adjustable range of opticalpower that is based in part on adjusting a curvature of the deformablemembrane. Enclosed between the transparent substrate layer and thedeformable membrane is a liquid (in some embodiments may be a constantvolume). The plurality of optical states of the LC lens and theadjustable range of optical power of the liquid lens structure togetherprovide a continuous range of adjustment of optical power for thevarifocal block.

The varifocal block may be part of a head-mounted display (HMD). The HMDpresents content via an electronic display to a wearing user at a focaldistance. The varifocal block presents the content over a plurality ofimage planes that are associated with different optical powers of thevarifocal block. As noted above, the varifocal block has a continuousrange of adjustment of optical power. Each value of optical power overthe continuous range of adjustment of optical power corresponds to adifferent image plane of the plurality of image planes. In someembodiments, the varifocal block adjusts the image plane location inaccordance with instructions from the HMD to, e.g., mitigate vergenceaccommodation conflict of eyes of the wearing user. The image planelocation is adjusted by adjusting an optical power associated with thevarifocal block, and specifically by adjusting the optical powersassociated with one or both of the liquid lens structure and the LClens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the relationship between vergence and eye focal length inthe real world.

FIG. 1B shows the conflict between vergence and eye focal length in athree-dimensional display screen.

FIG. 2A is a wire diagram of a head-mounted display, in accordance withan embodiment.

FIG. 2B is a cross section of a front rigid body of the head-mounteddisplay in FIG. 2A, in accordance with an embodiment.

FIG. 3A is an example Pancharatnam Berry Phase liquid crystal lens,according to an embodiment.

FIG. 3B is an example of liquid crystal orientations in the PancharatnamBerry Phase liquid crystal lens of FIG. 3A, according to an embodiment.

FIG. 3C is a portion of liquid crystal orientations in the PancharatnamBerry Phase liquid crystal lens of FIG. 3A, according to an embodiment.

FIG. 4 is a diagram of a varifocal structure including an active PBPliquid crystal lens, according to an embodiment

FIG. 5 is a diagram of a varifocal structure including a passive PBPliquid crystal lens, according to an embodiment.

FIG. 6 is a diagram of another example of a varifocal structureincluding a passive PBP liquid crystal lens, according to an embodiment

FIG. 7 is varifocal system in which a HMD operates, according to anembodiment.

FIG. 8 is a process for mitigating vergence-accommodation conflict byadjusting the focal length of a HMD, according to an embodiment.

FIG. 9 shows an example process for mitigating vergence-accommodationconflict by adjusting a focal length of a varifocal block that includesvarifocal structures, in accordance with an embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Configuration Overview

A varifocal system includes a head-mounted display (HMD). The HMDincludes a varifocal block. The HMD presents content via an electronicdisplay to a wearing user at a focal distance. The varifocal blockadjusts the focal distance in accordance with instructions from the HMDto, e.g., mitigate vergence accommodation conflict of eyes of thewearing user. The focal distance is adjusted by adjusting an opticalpower associated with the varifocal block, and specifically by adjustingthe optical powers associated with one or more varifocal structureswithin the varifocal block.

A varifocal structure is an optical device that is configured todynamically adjust its focus in accordance with instructions from thevarifocal system. A varifocal structure includes at least a PancharatnamBerry Phase (PBP) liquid crystal lens and a liquid lens structure inoptical series. Optical series refers to relative positioning of aplurality of optical elements such that light, for each optical elementof the plurality of optical elements, is transmitted by that opticalelement before being transmitted by another optical element of theplurality of optical elements. Moreover, ordering of the opticalelements does not matter. For example, optical element A placed beforeoptical element B, or optical element B placed before optical element A,are both in optical series. Similar to electric circuitry design,optical series represents optical elements with their optical propertiescompounded when placed in series.

A PBP liquid crystal lens may be active or passive. An active PBP liquidcrystal lens is an optical element that has three discrete focal states(also referred to as optical states). The three optical states are anadditive state, a neutral state, and a subtractive state. The additivestate adds optical power to the system (i.e., has a positive focus of‘f’), the neutral state does not affect the optical power of the system(and does not affect the polarization of light passing through the PBPliquid crystal lens), and the subtractive state subtracts optical powerfrom the system (i.e., has a negative focus of ‘−f’). The state of anactive PBP liquid crystal lens is determined by the by the handedness ofpolarization of light incident on the active PBP liquid crystal lens andan applied voltage. An active PBP liquid crystal lens operates in asubtractive state responsive to incident light with a right handedcircular polarization and an applied voltage of zero (or more generallybelow some minimal value), operates in an additive state responsive toincident light with a left handed circular polarization and the appliedvoltage of zero (or more generally below some minimal value), andoperates in a neutral state (regardless of polarization) responsive toan applied voltage larger than a threshold voltage which aligns liquidcrystal with positive dielectric anisotropy along with the electricfield. Note that if the active PBP liquid crystal lens is in theadditive or subtractive state, light output from the active PBP liquidcrystal lens has a handedness opposite that of the light input into theactive PBP liquid crystal lens. In contrast, if the active PBP liquidcrystal lens is in the neutral state, light output from the active PBPliquid crystal lens has the same handedness as the light input into theactive PBP liquid crystal lens.

In contrast, a passive PBP liquid crystal lens has two optical states,specifically, an additive state and a subtractive state. And the stateof a passive PBP liquid crystal lens is determined by the handedness ofpolarization of light incident on the passive PBP liquid crystal lens. Apassive PBP liquid crystal lens operates in a subtractive stateresponsive to incident light with a right handed polarization andoperates in an additive state responsive to incident light with a lefthanded polarization. Note that the passive PBP liquid crystal lensoutputs light that has a handedness opposite that of the light inputinto the passive PBP liquid crystal lens.

The liquid lens structure is an optical element that is able to adjustfocus (i.e., optical power) over a continuous range from a positivevalue to a negative value. There are fixed volume fluid filled lenses,and variable volume fluid-filled lenses. For HMD applications, a liquidlens can be preferable for many reasons. For example, a liquid lensoffers a compact design, a large clear aperture size and a stableoptical performance within the variable focus range (e.g. no air bubble,freedom on the frame/lens shape). In this application, we specificallyuse the liquid membrane lens which has a continuous range of 0 to F (interms of optical power this may be represented as 0 to D. Note that anactive PBP liquid crystal lens can adjust focus by −f, 0, or f, or interms of optical power −d, 0, or d (here d is a positive number).Accordingly, in some embodiments, the varifocal structure that includesan active PBP liquid crystal lens is able to dynamically impart acontinuous tunability of optical power ranging from −d to (d+D). Inalternate embodiments, a varifocal structure may include a passive PBPliquid crystal lens that has a continuous tunability of optical powerranging from −d to (D−d); or d to (d+D).

In some embodiments, a virtual object is presented on the electronicdisplay of the HMD that is part of the varifocal system. The lightemitted by the HMD is configured to have a particular focal distance,such that the virtual scene appears to a user at a particular focalplane. As the content to be rendered moves closer/farther from the user,the HMD correspondingly instructs the varifocal block to adjust thefocal distance to mitigate a possibility of a user experiencing aconflict with eye vergence and eye accommodation. Additionally, in someembodiments, the HMD may track a user's eyes such that the varifocalsystem is able to approximate gaze lines and determine a gaze pointincluding a vergence depth (an estimated point of intersection of thegaze lines) to determine an appropriate amount of accommodation toprovide the user. The gaze point identifies an object or plane of focusfor a particular frame of the virtual scene and the HMD adjusts thedistance of the varifocal block to keep the user's eyes in a zone ofcomfort as vergence and accommodation change.

Vergence-Accommodation Overview

Vergence-accommodation conflict is a problem in many virtual realitysystems. Vergence is the simultaneous movement or rotation of both eyesin opposite directions to obtain or maintain single binocular vision andis connected to accommodation of the eye. Under normal conditions, whenhuman eyes look at a new object at a distance different from an objectthey had been looking at, the eyes automatically change focus (bychanging their shape) to provide accommodation at the new distance orvergence depth of the new object. FIG. 1A shows an example of how thehuman eye experiences vergence and accommodation in the real world. Inthe example of FIG. 1A, the user is looking at a real object 100 (i.e.,the user's eyes are verged on the real object 100 and gaze lines fromthe user's eyes intersect at real object 100.). As the real object 100is moved closer to the user, as indicated by the arrow in FIG. 1A, eacheye 102 rotates inward (i.e., convergence) to stay verged on the realobject 100A. As the real object 100 gets closer, the eye 102 must“accommodate” for the closer distance by changing its shape to reducethe power or focal length. Thus, under normal conditions in the realworld, the vergence depth (d_(v)) equals the focal length (d_(f)).

FIG. 1B shows an example conflict between vergence and accommodationthat can occur with some three-dimensional displays. In this example, auser is looking at a virtual object 100B displayed on an electronicscreen 104; however, the user's eyes are verged on and gaze lines fromthe user's eyes intersect at virtual object 100B, which is a greaterdistance from the user's eyes than the electronic screen 104. As thevirtual object 100B is rendered on the electronic display 104 to appearcloser to the user, each eye 102 again rotates inward to stay verged onthe virtual object 100B, but the power or focal length of each eye isnot reduced; hence, the user's eyes do not accommodate as in FIG. 1A.Thus, instead of reducing power or focal length to accommodate for thecloser vergence depth, each eye 102 maintains accommodation at adistance associated with the electronic display 104. Thus, the vergencedepth (d_(v)) often does not equal the focal length (d_(f)) for thehuman eye for objects displayed on 3D electronic displays. Thisdiscrepancy between vergence depth and focal length is referred to as“vergence-accommodation conflict.” A user experiencing only vergence oraccommodation and not both will eventually experience some degree offatigue and nausea, which is undesirable for virtual reality systemcreators.

Head-Mounted Display Overview

FIG. 2A is a wire diagram of a HMD 200, in accordance with anembodiment. The HMD 200 includes a front rigid body 205 and a band 210.The front rigid body 205 includes one or more electronic displayelements of an electronic display (not shown), an IMU 215, the one ormore position sensors 220, and the locators 225. In the embodiment shownby FIG. 2A, the position sensors 220 are located within the IMU 215, andneither the IMU 215 nor the position sensors 220 are visible to theuser. The IMU 215, the position sensors 220, and the locators 225 arediscussed in detail below with regard to FIG. 7. Note in embodiments,where the HMD 200 acts as an AR or MR device portions of the HMD 200 andits internal components are at least partially transparent.

FIG. 2B is a cross section 250 of the front rigid body 205 of theembodiment of the HMD 200 shown in FIG. 2A. As shown in FIG. 2B, thefront rigid body 205 includes an electronic display 255 and a varifocalblock 260 that together provide image light to an exit pupil 263. Theexit pupil 263 is the location of the front rigid body 205 where auser's eye 265 is positioned. For purposes of illustration, FIG. 2Bshows a cross section 250 associated with a single eye 265, but anothervarifocal block 260, separate from the varifocal block 260, providesaltered image light to another eye of the user. Additionally, the HMD200 includes an eye tracking system (not shown). The eye tracking systemmay include, e.g., one or more sources that illuminate one or both eyesof the user, and one or more cameras that captures images of one or botheyes of the user.

The electronic display 255 displays images to the user. In variousembodiments, the electronic display 255 may comprise a single electronicdisplay or multiple electronic displays (e.g., a display for each eye ofa user). Examples of the electronic display 255 include: a liquidcrystal display (LCD), an organic light emitting diode (OLED) display,an active-matrix organic light-emitting diode display (AMOLED), a QOLED,a QLED, some other display, or some combination thereof.

The varifocal block 260 adjusts an orientation from light emitted fromthe electronic display 255 such that it appears at particular focaldistances from the user. The varifocal block 260 includes one or morevarifocal structures in optical series. A varifocal structure is anoptical device that is configured to dynamically adjust its focus inaccordance with instructions from a varifocal system. A varifocalstructure includes at least a PBP liquid crystal lens (passive oractive) and a liquid lens structure. The varifocal structure may alsoinclude one or more substrate layers, a switchable half waveplate(SHWP), a circular polarizer, or some combination thereof. Details ofPBP liquid crystal lenses are discussed in detail below with regard toFIGS. 3A-3C. And different embodiments, of varifocal structures arediscussed in detail below with regard to FIGS. 4-6.

A SHWP is a half waveplate that transmits a particular handedness ofpolarized light in accordance with a switching state (i.e., active ornon-active). A varifocal block may use the SHWP to control thehandedness of polarization of light in accordance with a switchingstate. The switching state of a SHWP is either active or non-active.When active, the SHWP reverses the handedness of polarized light, andwhen non-active, the SHWP transmits polarized light without affectingthe handedness. Recall that a PBP lens acts in an additive state if itreceives right handed circularly polarized light, and conversely, actsin a subtractive state if it receives left handed circularly polarizedlight. Accordingly, a SHWP placed before a PBP liquid crystal lens inoptical series is able to control whether the PBP liquid crystal lensacts in an additive or subtractive state by controlling the handednessof polarization of the light incident on the PBP liquid crystal lens.

A circular polarizer polarized light converts incident light to circularpolarized light of a particular handedness (i.e., right or left). Thecircular polarizer is transmissive such that it outputs circularlypolarized light. Note, a circular polarizer is a passive element, and issmaller and less complex than, e.g., a SHWP.

The substrate layers are layers which other elements (e.g., SHWP,tunable liquid lens, liquid crystal, etc.) may be formed upon, coupledto, etc. The substrate layers are substantially transparent in thevisible band (˜380 nm to 750 nm). And in some embodiments, may also betransparent in some or all of the infrared (IR) band (˜750 nm to 1 mm).The substrate layers may be composed of, e.g., SiO₂, plastic, sapphire,etc. These layers are discussed in more detail with regard to FIGS. 4-6.

The liquid lens structure is an optical element that is able to adjustfocus over a continuous range of 0 to F (e.g., 0 to 2 Diopters), where Fis an upper focal limit. The liquid lens structure includes a liquidlayer that is encapsulated between a transparent deformable membranelayer and a transparent rigid layer. A varifocal system is able todynamically control a location of a focal distance of the liquid lensstructure. In some embodiments, the substrate layer is flat, however, itmay also be curved. For example, the substrate may be a convex substrateor a concave substrate, both of which may adjust optical power (increaseor decrease), but at the expense of increasing total thickness. Theliquid layer includes one or more liquids which are substantiallytransparent the optical band of interest (e.g., visible, IR< etc.).There are two types of fluid-filled liquid membrane lens: constant fluidvolume and variable fluid volume. The electro-wetting liquid lens isgenerally not applicable for HMDs because of the high power consumptionand limited clear aperture size. A constant fluid volume liquid membranelens is preferred in the HMD system, because of the compact designrequirements. A constant fluid volume liquid membrane lens has a fixedvolume of fluid which is encapsulated between a transparent deformablemembrane layer and a rigid transparent substrate. To tune the focus, oneportion of the membranes moves downward, other portion of the membranesmoves upwards and form a lens of variable powers.

Additionally, in some embodiments, the varifocal block 260 magnifiesreceived light, corrects optical errors associated with the image light,and presents the corrected image light is presented to a user of the HMD200. The varifocal block 260 may additionally include one or moreoptical elements in optical series. An optical element may be anaperture, a Fresnel lens, a convex lens, a concave lens, a filter, orany other suitable optical element that affects the blurred image light.Moreover, the varifocal block 260 may include combinations of differentoptical elements. In some embodiments, one or more of the opticalelements in the varifocal block 260 may have one or more coatings, suchas anti-reflective coatings.

FIG. 3A is an example PBP liquid crystal lens 300, according to anembodiment. The PBP liquid crystal lens 300 creates a respective lensprofile via an in-plane orientation (θ, azimuth angle) of a liquidcrystal molecule, in which the phase difference T=2θ. In contrast, aconventional liquid crystal lens creates a lens profile via abirefringence (An) and layer thickness (d) of liquid crystals, and anumber (#) of Fresnel zones (if it is Fresnel lens design), in which thephase difference T=Δnd*#*2π/λ. Accordingly, in some embodiments, a PBPliquid crystal lens 300 may have a large aperture size and can be madewith a very thin liquid crystal layer, which allows fast switching speedto turn the lens power on/off.

Design specifications for HMDs used for VR, AR, or MR applicationstypically requires a large range of optical power to adapt for human eyevergence-accommodation (e.g., ˜±2 Diopters or more), fast switchingspeeds (e.g., ˜300 ms), and a good quality image. Note conventionalliquid crystal lenses are not well suited to these applications as, aconventional liquid crystal lens generally would require the liquidcrystal to have a relatively high index of refraction or be relativelythick (which reduces switching speeds). In contrast, a PBP liquidcrystal lens is able to meet design specs using a liquid crystal havinga relatively low index of refraction, is thin (e.g., a single liquidcrystal layer can be ˜2 μm), and has high switching speeds (e.g., 300ms).

FIG. 3B is an example of liquid crystal orientations 310 in the PBPliquid crystal lens 300 of FIG. 3A, according to an embodiment. In thePBP liquid crystal lens 300, an azimuth angle (θ) of a liquid crystalmolecule is continuously changed from a center 320 of the liquid crystallens 300 to an edge 330 of the PBP liquid crystal lens 300, with avaried pitch Λ. Pitch is defined in a way that the azimuth angle of LCis rotated 180° from the initial state.

FIG. 3C is a section of liquid crystal orientations 340 taken along a yaxis in the PBP liquid crystal lens 300 of FIG. 3A, according to anembodiment. It is apparent from the liquid crystal orientation 340 thata rate of pitch variation is a function of distance from the lens center320. The rate of pitch variation increases with distance from the lenscenter. For example, pitch at the lens center (Λ₀), is the slowest andpitch at the edge 320 (Λ_(r)) is the highest, i.e., Λ₀>Λ₁> . . . >Λ_(r).In the x-y plane, to make a PBP liquid crystal lens with lens radius (r)and lens power (+/−f), the azimuth angle needs to meet: 2θ=r²/f*(π/λ),where λ is the wavelength of light. Along with the z-axis, a dual twistor multiple twisted structure layers offers achromatic performance onefficiency in the PBP liquid crystal lens 300. Along with the z-axis,the non-twisted structure is simpler to fabricate then a twistedstructure, but is optimized for a monochromatic light.

Note that a PBP liquid crystal lens may have a twisted or non-twistedstructure. In some embodiments, a stacked PBP liquid crystal lensstructure may include one or more PBP liquid crystal lenses having atwisted structure, one or more PBP liquid crystal lenses having anon-twisted structure, or some combination thereof.

Example Varifocal Structures

Below various designs of varifocal structures are discussed. It isimportant to note that these designs are merely illustrative, and otherdesigns of varifocal structures may be generated using the principlesdescribed herein. In some embodiments, the varifocal structures withinthe varifocal block 260 are designed to meet requirements for an HMD(e.g., the HMD 200). Design requirements may include, for example, largeaperture size (e.g., >4 cm) for large field of view (e.g., FOV, ˜90degrees with 20 mm eye relief distance), large optical power (e.g., ±2.0Diopters) for adapting human eye vergence—accommodation, and fastswitching speed (˜300 ms) is for adapting human eyevergence—accommodation, and good image quality for meeting human eyeacuity. In other embodiments the varifocal structures can include otheroptical elements in optical series.

FIG. 4 is a diagram of a varifocal structure 400 including an active PBPliquid crystal lens, according to an embodiment. The varifocal structure400 has a continuous focal range of −f to (F+f), or in terms ofdiopters, −d to (D+d), where D is the maximum lens power of the liquidmembrane lens module, and d is a fixed lens power of the PBP LC lensmodule, both D and d are positive integers. The varifocal structure 400includes a SHWP 410, a substrate layer 420, a PBP liquid crystal layer430, a solid substrate layer 440, a liquid layer 450, a membrane layer460, and an outer substrate layer 470 in optical series for thisembodiment.

The SHWP 410 is a half waveplate that reverses a handedness of polarizedlight in accordance with a switching state (i.e., active or non-active).In an active state the SHWP 410 reverses the handedness of incidentcircularly polarized light 480 (e.g., right to left, or vice versa). Ina non-active state, the SHWP 410 transmits the circularly polarizedlight 480 without affecting its handedness.

The substrate layer 420 is a substance that serves as a foundation foradding additional layers to the varifocal structure 400. The substratelayer 420 may be e.g., silicon, silicon dioxide, sapphire, plastic, orsome other semiconductor that is at least partially transmissive tolight emitted by an electronic display. In this embodiment, thesubstrate layer 420 is coupled to the SHWP 410.

The PBP liquid crystal layer 430 is an active PBP liquid crystal lens.The PBP liquid crystal layer 430 has three optical states, an additivestate, a subtractive state, and a neutral state. If not in the neutralstate, and right handed circularly polarized light is incident on thePBP liquid crystal layer 430, the PBP liquid crystal layer 430 acts inan additive state such that a focus of the PBP liquid crystal layer 430is f. In contrast, if not in the neutral state, and left handedcircularly polarized light is incident on the PBP liquid crystal layer430, the PBP liquid crystal layer 430 acts in a subtractive state suchthat a focus of the PBP liquid crystal layer 430 is −f. If in a neutralstate (i.e., an applied voltage is over a threshold value), the PBPliquid crystal layer 430 does not affect the net optical power of thevarifocal structure 400. Accordingly, the PBP liquid crystal layer 430is able to adjust focus by either −f, 0, or f—or in terms of diopter −d,0, d. In this embodiment, the PBP liquid crystal layer 430 is coupled tothe substrate layer 420.

The solid substrate layer 440 is a substance that serves as a foundationfor the liquid layer 450. The solid substrate layer 440 may be e.g.,silicon, silicon dioxide, sapphire, plastic, or some other semiconductorthat is at least partially transmissive to light emitted by anelectronic display (e.g., electronic display 255). In this embodiment,the middle substrate layer 420 is coupled to the PBP liquid crystallayer 430.

The liquid layer 450 includes one or more liquids. The one or moreliquids may include, e.g., water, oil, etc. The liquid layer 450 issubstantially transmissive (i.e., transparent) to light emitted by theelectronic display.

The membrane layer 460 encloses the liquid layer 450 between itself andthe substrate layer 420, and together form a liquid lens structure 465.A varifocal system controls voltage applied to the membrane layer 460 todynamically control, over a continuous range, an amount of optical powerassociated with the liquid lens structure 465. Responsive to an appliedvoltage, a top portion of the membrane layer 460 moves downward and abottom portion of the membrane layer 460 moves upwards which causescurvature in the membrane layer 460 and the enclosed liquid in theliquid layer 450 to cause a change in optical power. Accordingly, theliquid lens structure 465 can vary a position of the top and/or bottomportion of the membrane layer 460 to cause a variation in optical power(e.g., to adjust optical power by 0 to D). In this embodiment, theliquid layer 450 is coupled to the solid substrate layer 440. Note thatcolor dispersion of the membrane layer 460—liquid layer 450 is oppositeof color dispersion caused by the PBP liquid crystal layer 430, whichhelps to mitigate color dispersion in the circularly polarized light490. In this embodiment, the membrane layer 460 is coupled to the liquidlayer 450.

The outer substrate layer 470 is formed on the membrane layer 460 andprotects the membrane layer 460 from environmental factors (e.g.,oxygen, water, etc.). The outer substrate layer 470 is transparent tothe light emitted from the electronic display, and may be formed from,e.g., transparent glass, sapphire, plastic, some other material that istransparent to the light emitted by electronic display, or somecombination thereof.

The varifocal structure 400 is relatively thin (e.g., a thickness 485 of˜2.2 mm) making it useful for applications with a HMD or more generallydevices where a small form factor and weight are considerations. Thevarifocal structure 400 provides a continuous range of adjustment ofoptical power from −D to 2D (when D=d); a continuous range of adjustmentof optical power from −d to −d+D, and from d to d+D (when d≠D). Forexample, assuming the circularly polarized light 480 is right handedcircularly polarized light, this range of adjustment may be achievedusing, e.g., the settings shown in Table 1 below.

TABLE 1 Settings for Optical Power Range Optical Power Liquid Range PBPLC LENS Lens of the SHWP Optical Optical Varifocal Configuration # StatePower Power structure 1 Non- d (additive) From 0 to D d to d + D Active2 Active −d (subtractive) From 0 to D −d to −d + D 3 Non-active 0(Neutral) From 0 to D 0 to D or ActiveFor example, a liquid membrane lens offers a continuous tunable rangefrom 0 to 1.5 diopter power before getting in the gravity deformationdegradation in a vertically aligned configuration. An active PBP LC lenswith an SHWP (switchable half waveplate), which provides three discretestates, (e.g. −1.5, 0, 1.5 diopter). Then, the continuously tunablerange of the variable focal structure (400) is from −1.5 Diopter to 3Diopter. With a passive PBP LC lens attached with an SWHP, whichprovides two discrete states, (e.g., −1.5 diopter, 1.5 diopter), thecontinuously tunable range of the lens stack is from −1.5 Diopter to 0,and 1.5 Diopter to 3 Diopter. Using a passive PBP LC lens offers arelatively simpler fabrication cost than the active PBP LC lens, and athinner stack thickness. In a case, which the PBL LC lens has an opticalpower larger than the liquid membrane lens, (e.g. −3, 0, 3 diopters foran active PBP LC lens; −3, 3 diopters from a passive PBP LC lens), thecontinuously tunable range of the varifocal structure is from either −3Diopter to −1.5 Diopter, 0 Diopter to 1.5 Diopter, and 3 Diopter to 4.5Diopter for an active PBP LC lens or −3 Diopter to −1.5 Diopter and 3Diopter to 4.5 Diopter for a passive PBP LC lens.

Note that this embodiment is based on circularly polarized light 480being right handed circularly polarized light. In alternate embodiments,the circularly polarized light 480 may be left handed circularlypolarized light. In this case, the varifocal structure 400 operates insubstantially the same way, except the times when the SHWP 410 is activeor non-active would be reversed. In alternate embodiments (not shown).The PBP liquid crystal layer 430 is a passive PBP liquid crystal lens,and the varifocal structure 400 would not include the substrate layer420.

FIG. 5 is a diagram of a varifocal structure 500 including a passive PBPliquid crystal lens, according to an embodiment. The varifocal structure500 includes a circular polarizer 510, a PBP liquid crystal layer 520, asolid substrate layer 440, a liquid layer 450, a membrane layer 460, andan outer substrate layer 470 in optical series for this embodiment.

The circular polarizer 510 converts incident light (light in 530) tocircular polarized light of a particular handedness (i.e., right orleft). Note, a circular polarizer is a passive element, and is smallerand less complex than, e.g., the SHWP 410. Additionally, the circularpolarizer 510 is fixed in terms of what handedness of polarization itoutputs. Accordingly, in some embodiments, the circular polarizer 510converts light in 530 to right handed circularly polarized light, and inalternate embodiments, the circular polarizer 510 converts the light in530 to left handed circularly polarized light.

The PBP liquid crystal layer 520 is a passive PBP liquid crystal lens.The PBP liquid crystal layer 520 has two optical states: an additivestate and a subtractive state. For a PBP liquid crystal layer 520 whichacts in an additive state (focus at f, add d to the optical power) forright handed circularly polarized incident light, the PBP liquid crystallayer 520 acts in a subtractive state in contrast for left handedcircularly polarized incident light (focus at −f, subtracts d from theoptical power). Accordingly, depending on the handedness of polarizationof incident light—which depends on the handedness of the circularpolarizing filter 510, the PBP liquid crystal layer 520 is able toadjust focus by either −f or f—or in terms of diopter −d or d. In thisembodiment, the PBP liquid crystal layer 520 is coupled to the circularpolarizer 510. The solid substrate layer 440, the liquid layer 450, themembrane layer 460, and the outer substrate layer 470, operatesubstantially the same as described above with reference to FIG. 4.

In some embodiments, the varifocal structure 500 provides a continuousrange of adjustment of optical power from −d to (D−d) if the circularpolarizer 510 outputs left handed circularly polarized light. Lefthanded circularly polarized light causes the PBP crystal layer 520 toremove diopters of optical power. The liquid lens structure 465 is ableto add back anywhere from 0 to D optical power—meaning the net tunablerange of the varifocal structure 500 is −d to (D−d). In embodiments,where D equals d, this would provide a −d to 0 continuous range oftunability of optical power.

In alternate embodiments, the varifocal structure 500 provides acontinuous range of adjustment of optical power from d to (d+D) if thecircular polarizer 510 outputs right handed circularly polarized light.Right handed circularly polarized light causes the PBP crystal layer 520to add diopters of optical power. The liquid lens structure 465 is ableto add an additional anywhere from 0 to D optical power—meaning that inthis embodiment, the net tunable range of the varifocal structure 500 isd to (d+D). Note, in some embodiments, the liquid lens structure 465 maybe configured to instead adjust optical power from −D to 0. In thiscase, the net tunable range of the varifocal structure 500 is (d−D) tod. In embodiments, where D equals d, this would provide a 0 to dcontinuous range of tunability of optical power. Note—in someembodiments, the PBP liquid crystal layer 520 is an active PBP liquidcrystal lens, and by placing the PBP liquid crystal layer 520 in aneutral state would provide tenability from 0 to D.

Note that use of passive elements (i.e., the circular polarizer 510 andthe PBP liquid crystal layer 520) result in the varifocal structurebeing substantially thinner than the varifocal structure 400. Forexample, a thickness 560 of the varifocal structure 500 may beapproximately 1.7 mm.

FIG. 6 is a diagram of another example of a varifocal structure 600including a passive PBP liquid crystal lens, according to an embodiment.The varifocal structure 600 includes a solid substrate layer 440, aliquid layer 450, a membrane layer 460, a substrate layer 410, acircular polarizer 510, and a PBP liquid crystal layer 610 in opticalseries for this embodiment.

The solid substrate layer 440, the liquid layer 450, the membrane layer460, the substrate layer 410, the circular polarizer 510, operatesubstantially the same as described above with reference to FIG. 4. ThePBP liquid crystal layer 610 is substantially the same as the PBP liquidcrystal layer 520 (e.g., is as a passive PBP liquid crystal lens),except that it also includes an outer surface 615 that acts in a mannersimilar to the outer substrate layer 470. The outer surface 615 istransparent to the light emitted from the electronic display, and may beformed from, e.g., transparent glass, sapphire, plastic, some othermaterial that is transparent to the light emitted by electronic display,or some combination thereof.

The varifocal structure 600 provides a same range of adjustment ofoptical power as the varifocal structure 500. For example, in someembodiments, the varifocal structure 600 provides a continuous range ofadjustment of optical power from −d to (D−d) if the circular polarizer510 outputs left handed circularly polarized light. In alternateembodiments, the varifocal structure 600 provides a continuous range ofadjustment of optical power from d to (d+D) if the circular polarizer510 outputs right handed circularly polarized light.

Note that use of passive elements (i.e., the circular polarizer 510 andthe PBP liquid crystal layer 520) result in the varifocal structurebeing substantially thinner than the varifocal structure 400. Forexample, a thickness 620 of the varifocal structure 600 may beapproximately 1.7 mm.

System Overview

FIG. 7 is varifocal system 700 in which a HMD 705 operates. Thevarifocal system 700 may be for use as a virtual reality (VR) system, anaugmented reality (AR) system, a mixed reality (MR) system, or somecombination thereof. In this example, the varifocal system 700 includesa HMD 705, an imaging device 710, and an input interface 715, which areeach coupled to a console 720. While FIG. 7 shows a single HMD 705, asingle imaging device 710, and a single input interface 715, in otherembodiments, any number of these components may be included in thesystem. For example, there may be multiple HMDs 400 each having anassociated input interface 715 and being monitored by one or moreimaging devices 460, with each HMD 705, input interface 715, and imagingdevices 460 communicating with the console 720. In alternativeconfigurations, different and/or additional components may also beincluded in the varifocal system 700. The HMD 705 may act as a VR, AR,and/or a MR HMD. An MR and/or AR HMD augments views of a physical,real-world environment with computer-generated elements (e.g., images,video, sound, etc.).

The HMD 705 presents content to a user. In some embodiments, the HMD 705is an embodiment of the HMD 200 described above with reference to FIGS.2A and 2B. Example content includes images, video, audio, or somecombination thereof. Audio content may be presented via a separatedevice (e.g., speakers and/or headphones) external to the HMD 705 thatreceives audio information from the HMD 705, the console 720, or both.The HMD 705 includes an electronic display 255 (described above withreference to FIG. 2B), a varifocal block 260 (described above withreference to FIG. 2B), an eye tracking module 725, a vergence processingmodule 730, one or more locators 225, an internal measurement unit (IMU)215, head tracking sensors 735, and a scene rendering module 740.

The varifocal block 260 adjusts its focal length by adjusting a focallength of one or more varifocal structures. As noted above withreference to FIG. 2B-6B, the varifocal block 260 adjusts its focallength by activating and/or deactivating a SHWP, controlling a state ofa PBP liquid crystal lens, adjusting a liquid lens structure, somecombination thereof. The varifocal block 260 adjusts its focal lengthresponsive to instructions from the console 720. Note that a varifocaltuning speed of a varifocal structure is limited by a tuning speed ofthe liquid lens structure, and accordingly, the liquid lens structure iselectrically tuned.

The eye tracking module 725 tracks an eye position and eye movement of auser of the HMD 705. A camera or other optical sensor (that is part theeye tracking module 725) inside the HMD 705 captures image informationof a user's eyes, and eye tracking module 725 uses the capturedinformation to determine interpupillary distance, interocular distance,a three-dimensional (3D) position of each eye relative to the HMD 705(e.g., for distortion adjustment purposes), including a magnitude oftorsion and rotation (i.e., roll, pitch, and yaw) and gaze directionsfor each eye. In one example, infrared light is emitted within the HMD705 and reflected from each eye. The reflected light is received ordetected by the camera and analyzed to extract eye rotation from changesin the infrared light reflected by each eye. Many methods for trackingthe eyes of a user can be used by eye tracking module 725. Accordingly,the eye tracking module 725 may track up to six degrees of freedom ofeach eye (i.e., 3D position, roll, pitch, and yaw) and at least a subsetof the tracked quantities may be combined from two eyes of a user toestimate a gaze point (i.e., a 3D location or position in the virtualscene where the user is looking). For example, the eye tracking module725 integrates information from past measurements, measurementsidentifying a position of a user's head, and 3D information describing ascene presented by the electronic display 255. Thus, information for theposition and orientation of the user's eyes is used to determine thegaze point in a virtual scene presented by the HMD 705 where the user islooking.

The vergence processing module 730 determines a vergence depth of auser's gaze based on the gaze point or an estimated intersection of thegaze lines determined by the eye tracking module 725. Vergence is thesimultaneous movement or rotation of both eyes in opposite directions tomaintain single binocular vision, which is naturally and automaticallyperformed by the human eye. Thus, a location where a user's eyes areverged is where the user is looking and is also typically the locationwhere the user's eyes are focused. For example, the vergence processingmodule 730 triangulates the gaze lines to estimate a distance or depthfrom the user associated with intersection of the gaze lines. The depthassociated with intersection of the gaze lines can then be used as anapproximation for the accommodation distance, which identifies adistance from the user where the user's eyes are directed. Thus, thevergence distance allows determination of a location where the user'seyes should be focused.

The locators 225 are objects located in specific positions on the HMD705 relative to one another and relative to a specific reference pointon the HMD 705. A locator 225 may be a light emitting diode (LED), acorner cube reflector, a reflective marker, a type of light source thatcontrasts with an environment in which the HMD 705 operates, or somecombination thereof. Active locators 225 (i.e., an LED or other type oflight emitting device) may emit light in the visible band (˜380 nm to750 nm), in the infrared (IR) band (˜750 nm to 1 mm), in the ultravioletband (10 nm to 380 nm), some other portion of the electromagneticspectrum, or some combination thereof.

The locators 225 can be located beneath an outer surface of the HMD 705,which is transparent to the wavelengths of light emitted or reflected bythe locators 225 or is thin enough not to substantially attenuate thewavelengths of light emitted or reflected by the locators 225. Further,the outer surface or other portions of the HMD 705 can be opaque in thevisible band of wavelengths of light. Thus, the locators 225 may emitlight in the IR band while under an outer surface of the HMD 705 that istransparent in the IR band but opaque in the visible band.

The IMU 215 is an electronic device that generates fast calibration databased on measurement signals received from one or more of the headtracking sensors 735, which generate one or more measurement signals inresponse to motion of HMD 705. Examples of the head tracking sensors 735include accelerometers, gyroscopes, magnetometers, other sensorssuitable for detecting motion, correcting error associated with the IMU215, or some combination thereof. The head tracking sensors 735 may belocated external to the IMU 215, internal to the IMU 215, or somecombination thereof.

Based on the measurement signals from the head tracking sensors 735, theIMU 215 generates fast calibration data indicating an estimated positionof the HMD 705 relative to an initial position of the HMD 705. Forexample, the head tracking sensors 735 include multiple accelerometersto measure translational motion (forward/back, up/down, left/right) andmultiple gyroscopes to measure rotational motion (e.g., pitch, yaw, androll). The IMU 215 can, for example, rapidly sample the measurementsignals and calculate the estimated position of the HMD 705 from thesampled data. For example, the IMU 215 integrates measurement signalsreceived from the accelerometers over time to estimate a velocity vectorand integrates the velocity vector over time to determine an estimatedposition of a reference point on the HMD 705. The reference point is apoint that may be used to describe the position of the HMD 705. Whilethe reference point may generally be defined as a point in space, invarious embodiments, a reference point is defined as a point within theHMD 705 (e.g., a center of the IMU 630). Alternatively, the IMU 215provides the sampled measurement signals to the console 720, whichdetermines the fast calibration data.

The IMU 215 can additionally receive one or more calibration parametersfrom the console 720. As further discussed below, the one or morecalibration parameters are used to maintain tracking of the HMD 705.Based on a received calibration parameter, the IMU 215 may adjust one ormore of the IMU parameters (e.g., sample rate). In some embodiments,certain calibration parameters cause the IMU 215 to update an initialposition of the reference point to correspond to a next calibratedposition of the reference point. Updating the initial position of thereference point as the next calibrated position of the reference pointhelps reduce accumulated error associated with determining the estimatedposition. The accumulated error, also referred to as drift error, causesthe estimated position of the reference point to “drift” away from theactual position of the reference point over time.

The scene rendering module 740 receives content for the virtual scenefrom a VR engine 745 and provides the content for display on theelectronic display 255. Additionally, the scene rendering module 740 canadjust the content based on information from the vergence processingmodule 730, the IMU 215, and the head tracking sensors 735. The scenerendering module 740 determines a portion of the content to be displayedon the electronic display 255 based on one or more of the trackingmodule 755, the head tracking sensors 735, or the IMU 215, as describedfurther below.

The imaging device 710 generates slow calibration data in accordancewith calibration parameters received from the console 720. Slowcalibration data includes one or more images showing observed positionsof the locators 225 that are detectable by imaging device 710. Theimaging device 710 may include one or more cameras, one or more videocameras, other devices capable of capturing images including one or morelocators 225, or some combination thereof. Additionally, the imagingdevice 710 may include one or more filters (e.g., for increasing signalto noise ratio). The imaging device 710 is configured to detect lightemitted or reflected from the locators 225 in a field of view of theimaging device 710. In embodiments where the locators 225 includepassive elements (e.g., a retroreflector), the imaging device 710 mayinclude a light source that illuminates some or all of the locators 225,which retro-reflect the light towards the light source in the imagingdevice 710. Slow calibration data is communicated from the imagingdevice 710 to the console 720, and the imaging device 710 receives oneor more calibration parameters from the console 720 to adjust one ormore imaging parameters (e.g., focal length, focus, frame rate, ISO,sensor temperature, shutter speed, aperture, etc.).

The input interface 715 is a device that allows a user to send actionrequests to the console 720. An action request is a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.The input interface 715 may include one or more input devices. Exampleinput devices include a keyboard, a mouse, a game controller, or anyother suitable device for receiving action requests and communicatingthe received action requests to the console 720. An action requestreceived by the input interface 715 is communicated to the console 720,which performs an action corresponding to the action request. In someembodiments, the input interface 715 may provide haptic feedback to theuser in accordance with instructions received from the console 720. Forexample, haptic feedback is provided by the input interface 715 when anaction request is received, or the console 720 communicates instructionsto the input interface 715 causing the input interface 715 to generatehaptic feedback when the console 720 performs an action.

The console 720 provides content to the HMD 705 for presentation to theuser in accordance with information received from the imaging device710, the HMD 705, or the input interface 715. In the example shown inFIG. 4, the console 720 includes an application store 750, a trackingmodule 755, and the VR engine 745. Some embodiments of the console 720have different or additional modules than those described in conjunctionwith FIG. 4. Similarly, the functions further described below may bedistributed among components of the console 720 in a different mannerthan is described here.

The application store 750 stores one or more applications for executionby the console 720. An application is a group of instructions, that whenexecuted by a processor, generates content for presentation to the user.Content generated by an application may be in response to inputsreceived from the user via movement of the HMD 705 or the inputinterface 715. Examples of applications include gaming applications,conferencing applications, video playback application, or other suitableapplications.

The tracking module 755 calibrates the varifocal system 700 using one ormore calibration parameters and may adjust one or more calibrationparameters to reduce error in determining position of the HMD 705. Forexample, the tracking module 755 adjusts the focus of the imaging device710 to obtain a more accurate position for observed locators 225 on theHMD 705. Moreover, calibration performed by the tracking module 755 alsoaccounts for information received from the IMU 215. Additionally, iftracking of the HMD 705 is lost (e.g., imaging device 710 loses line ofsight of at least a threshold number of locators 225), the trackingmodule 755 re-calibrates some or all of the varifocal system 700components.

Additionally, the tracking module 755 tracks the movement of the HMD 705using slow calibration information from the imaging device 710 anddetermines positions of a reference point on the HMD 705 using observedlocators from the slow calibration information and a model of the HMD705. The tracking module 755 also determines positions of the referencepoint on the HMD 705 using position information from the fastcalibration information from the IMU 215 on the HMD 705. Additionally,the tracking module 755 may use portions of the fast calibrationinformation, the slow calibration information, or some combinationthereof, to predict a future location of the HMD 705, which is providedto the VR engine 745.

The VR engine 745 executes applications within the varifocal system 700and receives position information, acceleration information, velocityinformation, predicted future positions, or some combination thereof forthe HMD 705 from the tracking module 755. Based on the receivedinformation, the VR engine 745 determines content to provide to the HMD705 for presentation to the user, such as a virtual scene, one or morevirtual objects to overlay onto a real world scene, etc.

In some embodiments, the VR engine 745 maintains focal capabilityinformation of the varifocal block 260. Focal capability information isinformation that describes what focal distances are available to thevarifocal block 260. Focal capability information may include, e.g., arange of focus the varifocal block 260 is able to accommodate (e.g., 0to 4 diopters); combinations of settings for SHWPs (e.g., active ornon-active), active PBP liquid crystal lenses, and liquid tunable lensesthat map to particular focal planes; combinations of settings for PBPliquid crystal lenses and liquid tunable lenses that map to particularfocal planes; settings for liquid tunable lenses that map to particularfocal planes; or some combination thereof.

The VR engine 745 generates instructions for the varifocal block 260,the instructions causing the varifocal block 260 to adjust its focaldistance to a particular location. The VR engine 745 generates theinstructions based on focal capability information and, e.g.,information from the vergence processing module 730, the IMU 215, andthe head tracking sensors 735. The VR engine 745 uses the informationfrom the vergence processing module 730, the IMU 215, and the headtracking sensors 735, or some combination thereof, to select a focalplane to present content to the user. The VR engine 745 then uses thefocal capability information to determine settings for one or SHWPs, oneor more active PBP liquid crystal lenses, one or more liquid lensstructures, or some combination thereof, within the varifocal block 260that are associated with the selected focal plane. The VR engine 745generates instructions based on the determined settings, and providesthe instructions to the varifocal block 260.

Additionally, the VR engine 745 performs an action within an applicationexecuting on the console 720 in response to an action request receivedfrom the input interface 715 and provides feedback to the user that theaction was performed. The provided feedback may be visual or audiblefeedback via the HMD 705 or haptic feedback via VR input interface 715.

FIG. 8 is a process 800 for mitigating vergence-accommodation conflictby adjusting the focal length of an HMD 705, according to an embodiment.The process 800 may be performed by the varifocal system 700 in someembodiments. Alternatively, other components may perform some or all ofthe steps of the process 800. For example, in some embodiments, a HMD705 and/or a console (e.g., console 720) may perform some of the stepsof the process 800. Additionally, the process 800 may include differentor additional steps than those described in conjunction with FIG. 8 insome embodiments or perform steps in different orders than the orderdescribed in conjunction with FIG. 8.

As discussed above, a multifocal system 800 may dynamically vary itsfocus to bring images presented to a user wearing the HMD 200 intofocus, which keeps the user's eyes in a zone of comfort as vergence andaccommodation change. Additionally, eye tracking in combination with thevariable focus of the varifocal system 700 allows blurring to beintroduced as depth cues in images presented by the HMD 200.

The varifocal system 700 determines 810 a position, an orientation,and/or a movement of HMD 705. The position is determined by acombination of the locators 225, the IMU 215, the head tracking sensors735, the imagining device 710, and the tracking module 755, as describedabove in conjunction with FIG. 7.

The varifocal system 700 determines 820 a portion of a virtual scenebased on the determined position and orientation of the HMD 705. Thevarifocal system 700 maps a virtual scene presented by the HMD 705 tovarious positions and orientations of the HMD 705. Thus, a portion ofthe virtual scene currently viewed by the user is determined based onthe position, orientation, and movement of the HMD 705.

The varifocal system 700 displays 830 the determined portion of thevirtual scene being on an electronic display (e.g., the electronicdisplay 255) of the HMD 705. In some embodiments, the portion isdisplayed with a distortion correction to correct for optical error thatmay be caused by the image light passing through the varifocal block260. Further, the varifocal block 260 has activated/deactivated one ormore SHWPS, PBP liquid crystal lenses, one or more liquid lensstructures, or some combination thereof, to provide focus andaccommodation to the location in the portion of the virtual scene wherethe user's eyes are verged.

The varifocal system 700 determines 840 an eye position for each eye ofthe user using an eye tracking system. The varifocal system 700determines a location or an object within the determined portion atwhich the user is looking to adjust focus for that location or objectaccordingly. To determine the location or object within the determinedportion of the virtual scene at which the user is looking, the HMD 705tracks the position and location of the user's eyes using imageinformation from an eye tracking system (e.g., eye tracking module 725).For example, the HMD 705 tracks at least a subset of a 3D position,roll, pitch, and yaw of each eye and uses these quantities to estimate a3D gaze point of each eye.

The varifocal system 700 determines 850 a vergence depth based on anestimated intersection of gaze lines. For example, FIG. 9 shows a crosssection of an embodiment of the HMD 705 that includes camera 902 fortracking a position of each eye 265, the electronic display 255, and thevarifocal block 260 that includes two varifocal structures, as describedwith respect to FIGS. 2B and 4-6. In this example, the camera 902captures images of the user's eyes looking at an image object 908 andthe eye tracking module 725 determines an output for each eye 265 andgaze lines 906 corresponding to the gaze point or location where theuser is looking based on the captured images. Accordingly, vergencedepth (d_(v)) of the image object 908 (also the user's gaze point) isdetermined 850 based on an estimated intersection of the gaze lines 906.As shown in FIG. 9, the gaze lines 906 converge or intersect at distancedam, where the image object 908 is located. In some embodiments,information from past eye positions, information describing a positionof the user's head, and information describing a scene presented to theuser may also be used to estimate the 3D gaze point of an eye in variousembodiments.

Accordingly, referring again to FIG. 8, the varifocal system 700 adjusts860 an optical power of the HMD 705 based on the determined vergencedepth. The varifocal system 700 selects a focal plane that matches thevergence depth by controlling one or more SHWPs, one or more PBP liquidcrystal lenses, one or more liquid crystal lenses, or some combinationthereof. As described above, the optical power of the varifocal block260 is adjusted to change a focal distance of the HMD 705 to provideaccommodation for the determined vergence depth corresponding to whereor what in the displayed portion of the virtual scene the user islooking.

Additional Configuration Information

The foregoing description of the embodiments of the disclosure have beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. A varifocal block comprising: a liquid crystal(LC) lens that has an additive state that adds optical power to the LClens and a subtractive state that removes optical power from the LClens; and a liquid lens structure in optical series with the LC lens,the liquid lens structure having an adjustable range of optical power;wherein a range of adjustment of optical power for the varifocal blockis based in part on the additive state of the LC lens, the subtractivestate of the LC lens, and the adjustable range of optical power of theliquid lens structure.
 2. The varifocal block of claim 1, wherein theadjustable range of optical power of the liquid lens structure is acontinuous adjustable range of optical power.
 3. The varifocal block ofclaim 1, wherein the range of adjustment of optical power for thevarifocal block is a continuous range of adjustment of optical power. 4.The varifocal block of claim 1, further comprising a switchable halfwaveplate, and the LC lens is positioned between the liquid lensstructure and the switchable half waveplate.
 5. The varifocal block ofclaim 4, wherein, based in part on a state of the switchable halfwaveplate and a state of the LC lens, the range of adjustment of opticalpower for the varifocal block is selected from the group consisting of:d to (D+d), −d to (−d+D), and 0 to D, where: −d is the optical power ofthe LC lens in the subtractive state, D is a maximum optical power ofthe adjustable range of the liquid lens structure, and d is the opticalpower of the LC lens in the additive state.
 6. The varifocal block ofclaim 1, further comprising a circular polarizer, and the LC lens ispositioned between the liquid lens structure and the circular polarizer.7. The varifocal block of claim 1, further comprising a circularpolarizer that is positioned between the liquid lens structure and theLC lens.
 8. The varifocal block of claim 1, wherein the LC lens is aPancharatnam Berry Phase (PBP) liquid crystal lens.
 9. The varifocalblock of claim 1, wherein a color dispersion caused by the liquid lensstructure is opposite of a color dispersion caused by the LC lens. 10.The varifocal block of claim 1, wherein the liquid lens structurecomprises: a transparent substrate layer; a deformable membrane, whereinthe deformable membrane has an adjustable curvature; and a volume ofliquid enclosed between the transparent substrate layer and thedeformable membrane; wherein the adjustable range of optical power ofthe liquid lens structure is based in part on the adjustable curvatureof the deformable membrane.
 11. A head-mounted display (HMD) comprising:a varifocal block that receives content from an electronic display, andpresents the content over a plurality of image planes that areassociated with different optical powers of the varifocal block, thevarifocal block comprising: a liquid crystal (LC) lens that has anadditive state that adds optical power to the LC lens and a subtractivestate that removes optical power from the LC lens; and a liquid lensstructure in optical series with the LC lens, the liquid lens structurehaving an adjustable range of optical power; wherein a range ofadjustment of optical power for the varifocal block is based in part onthe additive state of the LC lens, the subtractive state of the LC lens,and the adjustable range of optical power of the liquid lens structure,and each value of optical power over the range of adjustment of opticalpower for the varifocal block corresponds to a different image plane ofthe plurality of image planes.
 12. The HMD of claim 11, wherein theadjustable range of optical power of the liquid lens structure is acontinuous adjustable range of optical power.
 13. The HMD of claim 11,wherein the range of adjustment of optical power for the varifocal blockis a continuous range of adjustment of optical power.
 14. The HMD ofclaim 11, further comprising one of: a switchable half waveplate, andthe LC lens is positioned between the liquid lens structure and theswitchable half waveplate; a first circular polarizer, and the LC lensis positioned between the liquid lens structure and the first circularpolarizer; and a second circular polarizer that is positioned betweenthe liquid lens structure and the LC lens.
 15. The HMD of claim 11,wherein the LC lens is a Pancharatnam Berry Phase (PBP) liquid crystallens.
 16. The HMD of claim 11, wherein a color dispersion caused by theliquid lens structure is opposite of a color dispersion caused by the LClens.
 17. The HMD of claim 11, wherein the liquid lens structurecomprises: a transparent substrate layer; a deformable membrane, whereinthe deformable membrane has an adjustable curvature; and a volume ofliquid enclosed between the transparent substrate layer and thedeformable membrane; wherein the adjustable range of optical power ofthe liquid lens structure is based in part on the adjustable curvatureof the deformable membrane.
 18. A method comprising: adjusting anoptical power of a varifocal block, comprising: selecting a liquidcrystal (LC) optical state from a plurality of optical states for a LClens of the varifocal block, the plurality of optical states includingan additive state that adds optical power to the LC lens and asubtractive state that removes optical power from the LC lens, selectingan optical power of a liquid lens structure of the varifocal block froman adjustable range of optical power, the liquid lens structure inoptical series with the LC lens, adjusting the LC lens to the selectedoptical state and the liquid lens structure to the selected opticalpower, and a combined optical power of the LC lens and the liquid lensstructure is the optical power of the varifocal block selected from arange of adjustment of optical power for the varifocal block that isbased in part on the additive state of the LC lens, the subtractivestate of the LC lens, and the adjustable range of optical power of theliquid lens structure.
 19. The method of claim 18, further comprising:presenting content from an electronic display of a head-mounted displayat an image plane associated with the optical power of the varifocalblock.
 20. The method of claim 18, wherein selecting the optical powerof the liquid lens structure from the adjustable range of opticalpowers, comprises: selecting a curvature for a deformable membrane ofthe liquid lens.